>Maybe while it's copying someone, it's exactly copying everything, knows what they know, but once it changes, it can no longer remember that version of itself because it's no longer structured that way

We can only go off what the film shows us.

And nothing suggests this in the film. It's relatively straight forward. It's perfect doppelgangers. Once they find out who is the Thing it doesn't make sense for it to switch back to that disguise.